An optical amplifier of the present disclosure includes an optical resonator that includes an amplification fiber capable of amplifying signal light having one or more propagation modes and resonates at least one propagation mode of the signal light amplified by the amplification fiber; an excitation light source that outputs excitation light for exciting the amplification fiber; and a multiplexer that multiplexes the signal light and the excitation light, in which the optical resonator includes a gain clamp setting unit which sets gain clamp for at least one propagation mode out of a plurality of propagation modes resonating in the optical resonator.

An optical device may include a package having a first port for receiving signal light, a source for providing pump light, a combiner for combining the signal light and the pump light into combined light, a second port for sending the combined light, a third port for receiving amplified light, and a free-space optical system for filtering amplified signal light from the amplified light, and a fourth port for sending the amplified signal light. The free-space optical system may include beam shaping optics that enlarge a beam size of the amplified light prior to the filtering.

Techniques for improving gain equalization in C- and L-band (C+L) erbium-doped fiber amplifier (EDFAs) are provided. For example, the C- and L-band amplification sections of a C+L EDFA may be separated and configured in a parallel arrangement or a serial arrangement. For both the parallel and serial arrangements, the C- and L-band amplification sections may share a common gain flattening filter (GFF) or each amplification section may include and employ a separate GFF. Moreover, in some examples, an interstage L-band GFF may be located before or upstream of the L-band amplification section such that the L-band optical signal is gain-equalized or flattened prior to the L-band amplification section amplifying the L-band.

Aspects of the present disclosure describe systems, methods, and structures for providing C-band and L-band transmission exhibiting increased power efficiency by diverting a portion of C-band optical energy to an input of L-band optical amplifiers (C-seeding) while optionally employing circulators to eliminate the need for optical isolators.

A Raman pump system for a Raman amplifier includes a plurality of primary Raman pumps each at a corresponding wavelength; and at least one pair of redundant Raman pumps including a primary redundant Raman pump at a primary wavelength and a secondary redundant Raman pump at a secondary wavelength, wherein only one of the primary redundant Raman pump and the secondary redundant Raman pump is employed based on a zero dispersion wavelength location of a fiber over which the Raman pump system operates. The secondary wavelength can be separated from the primary wavelength by at least 2 nm or 3 nm and no more than 10 nm. The Raman pump system can provide amplification across both the C Band and the L Band.

A dual output semiconductor optical amplifier-based tunable fiber laser is provided that can be switched from low to high power and vice versa. The laser system uses bidirectional semiconductor optical amplifier (SOA) for amplification and hence is able to introduce a unique feature of adjustable dual/single output ports.

[Problem] To accommodate single-band signal processing devices in a high-density manner.

[Solution] Provided is a system including: a first signal cable; a second signal cable; a third signal cable; a fourth signal cable; a first multi-band signal processing device that processes a first signal input from the first signal cable and outputs a resultant second signal to the second signal cable; a second multi-band signal processing device that processes a third signal input from the third signal cable and outputs a resultant fourth signal to the fourth signal cable; a first joint box that accommodates the first signal cable, the first multi-band signal processing device, the second signal cable, and the fourth signal cable; and a second joint box that accommodates the second signal cable, the third signal cable, the second multi-band signal processing device, and the fourth signal cable.

An amplification fiber includes a core which is doped with an erbium ion and a cladding which surrounds the core and has a refractive index lower than a refractive index of the core, and a relative refractive index difference ?n.sub.51 between the core and the cladding is not more than a smaller one of values of a relative refractive index difference ?n.sub.1 expressed as a predetermined expression related to a radius a of the core and a relative refractive index difference ?n.sub.2 expressed as a predetermined expression related to the radius a of the core.

Aspects of the present disclosure describe systems, methods, and structures for providing C-band and L-band transmission exhibiting increased power efficiency by diverting a portion of C-band optical energy to an input of L-band optical amplifiers (C-seeding) while optionally employing circulators to eliminate the need for optical isolators.

Aspects of the present disclosure describe systems, methods, and structures for providing bidirectional C-band and L-band transmission employing optical circulators which advantageously eliminates C\L WDM couplers while still blocking any backward amplified spontaneous emissions from optical amplifiers.